Vehicles, such as trucks, comprising hoist mechanisms for on-loading, off-loading, transporting, and dumping containers are known. One type of hoist mechanism that is commonly used is known as a “hook hoist.” A hook hoist may typically include an L-shaped hoist arm that is pivotally mounted to a tilt frame on the rear of the vehicle trailer or chassis. Part of the arm, often referred to as a “jib,” may be telescopically extendable and have a hook mounted to the outboard end thereof configured to engage with a transportable container. The tilt frame is itself pivotally mounted to the rear end of the vehicle chassis or sub-frame. One or more hydraulic cylinders are usually connected between the vehicle chassis or sub-frame and the hoist arm. Typically, the hook hoist mechanism will include a locking system operative to lock the hoist arm to the tilt frame and unlock the hoist arm from the tilt frame. Thus, when the hoist arm is not locked to the tilt frame, the hydraulic cylinder(s) can be actuated to pivot the hoist arm about its pivotal connection with the tilt frame, which may remain stationary. Often, the hoist arm can be pivoted toward the rear of the vehicle approximately 150° so that it may engage with a container and then be pivoted back, thereby lifting the container up onto the vehicle chassis. Subsequent extension of the jib may pull the container toward the front of the vehicle to a desired transport position. When the tilt frame is locked with the hoist arm, however, the hydraulic cylinder(s) can be actuated to pivot the tilt frame and hoist arm together as a unit about the pivotal mount between the tilt frame and the vehicle chassis or sub-frame. This action allows the operator to dump a container that is connected with the hook hoist on the vehicle.
As is known, if the system locking the hoist arm to the tilt frame becomes disengaged or otherwise breaks while the hoist arm and tilt frame are pivoted upwardly with a container, the whole assembly may come crashing down, likely causing considerable damage to the vehicle, hook hoist, and/or container. Various safety mechanisms have been designed to prevent this occurrence. Additional background regarding vehicle mounted hook hoists is provided in U.S. Pat. Nos. 5,542,807; 5,531,559; and 5,088,875, the entire disclosures of which are incorporated herein by reference for all purposes.